If you work your way backwards
through the fossil record, for about 400 million years you would
be working back through ages that included organisms living on
land. From about 400 million years back to 600 million
years, all kinds of complex multicellular life would have been
confined to the waters of the earth. From about 600 million
years ago back to about two billion years, youd be looking at
simple eukaryotes, and from two billion back to at least
3-and-a-half billion years, maybe much further, there would be evidence of
prokaryotes, including the stromatolites
mentioned in a later section. The earth itself is thought to have come
together as a planet a bit more than 4-and-a-half billion years
ago, but would have been in the finishing-up phase for perhaps
three-quarters of a billion years. So where did those
first prokaryotes come from?
One theory, panspermia or the
space seed hypothesis, proposes that life can be found on
meteors and other space debris. In fact, some very rigorous
tests suggest that there may be bacteria in space, perhaps left
from some long-destroyed planet and perhaps capable of surviving a
leisurely trip across the universe. If this is the case,
then no one could be sure of the conditions under which that otherworldly life
would have evolved. This is a possibility, but a bit of a
dead end explanations-wise.
Another theory is the special creation
theory, that supernatural forces (pick a candidate from a
long list) put the first prokaryotes together as part of the long
building plans which would ultimately lead to us. Science
tends to be resistant to supernatural explanations of things (they arent
really testable, and their proponents dont accept them as falsifiable), but
when youre dealing with conditions so far in the past, there is really no more or less evidence
for this idea than any of the others.

What we are going to look at are the theories
that assume that Earths life is "home-grown," emerging
from those earliest conditions to exist eventually as what we see
around us.

WHAT IS LIFE?

At their simplest beginnings, living
"things" really only needed three
characteristics: the ability to self-organize,
the ability to reproduce themselves and the ability
to evolve.

Eventually, to develop into the life we see on earth today, it needed a few
more traits: a DNA-based information coding system, a
complex protein-based chemistry, and a cell membrane
containment system. Put everything together and you have a simple prokaryote.

When biologists first began to wrestle with
these concepts, the ideas of uniformitarianism were still
powerful, and people theorized in term of a past world not all that
different from the world they knew. Our worlds ecosystems
depend upon photosynthesis to construct the fuel that all life
runs on; in an ancient world with conditions similar to
todays, you would need plants (as organisms that can make complex
"fuel" molecules using simple building blocks and energy available from
the environment, plants are known as one type of autotrophs,
or "self-feeders") to evolve first, or there would be no
bottom link to the food chain. This was an insurmountable
problem to theorists, because the processes of photosynthesis are
much too complicated to have spontaneously formed from
"nothing" under present-day conditions (or any
conditions).

But through the first half of the 20th Century,
studies from astronomy and geology suggested that the very early
Earth was a dramatically different place than it is today.
It was a hotter and nastier place, heated by a warmer sun and a
warmer interior. This suggests more violent weather, with
huge storms and lots of lightning. The atmosphere had no or
almost no oxygen, so no ozone layer to absorb the suns
ultraviolet rays. Simple organic materials, which are
common
in the space dust and debris that the Earth formed from, would
have filled the oceans with a kind of brownish gunk, full of potential
building blocks of life. This is known as primordial
soup, and this concept of the early Earth led to something
called theheterotroph hypothesis (heterotrophs
require their complex fuel molecules already made, unlike autotrophs).

THE HETEROTROPH
HYPOTHESIS

The strength of the heterotroph hypothesis is that
it gives the first forms of life a source of "food" that
doesnt itself come from living things: the primordial
soup. This is how the rest is supposed to have happened...
The ability to self-organize. This
requires some already-formed building blocks, from the soup, and a
source of energy that would serve to help drive them into
increasingly complex forms. Experiments
in the early 1950s began to confirm that such processes could
at least begin. Those
experiments used
simulated lightning, confined
to a "primordial soup bottle," to stimulate production
of complex materials. Since that time, all of the various
forms of energy available on the early earth have been tested,
with varying results. The current
"leading
contender" for life-organizer are Hydrothermal vents,
openings between Earths surface plates at the bottom of the
oceans. There, water mixes with hot magma and releases a hot
soup of materials even today. They have an energy source -
heat - a source of materials - once soup, now magma - and, perhaps
most importantly, are a stable, long-lasting ecosystem and a place
to "work the bugs out" of the earliest living systems.

The ability to reproduce.As
these early self-organizing molecules grew, only those which could
make and spread copies of themselves had any real future.
Life on todays Earth uses DNA code to store all of the
information it needs to make the proteins it actually runs on, but
DNA has little activity beyond that, and proteins generally cant
duplicate themselves. This means the first systems probably werent
DNA or protein molecules. There are theories that try to address
this problems, but the leading current theory is that the first
really complex systems were of RNA, a hypothesis
usually called the
RNA World hypothesis. RNA
has DNAs coding abilities and some protein-like activity, and it
isnt difficult to see the evolution of a DNA-coded protein system
growing quickly from an RNA ancestor.
The ability to evolve. Once you
have a planetwide ocean full of self-organizing molecules able to
reproduce themselves, you have a competitive ecosystem where
selection can take place. This stage of molecular
evolution would have favored those who could work most
efficiently, or best accumulate building blocks, or reproduce the
fastest, or work with other molecules in a cooperative fashion,
perhaps linking RNA or DNA codes for particularly good proteins
together to work as a unified system. And these unified
systems might work even better with some confinement and
protection...

THE RISE OF CELLS

That ancient primordial soup would have been coated
with a mixture of oily-fatty lipid substances, materials
that in a turbulent surf environment can form cell-like bubbles - not as
good a container as the membrane of actual cells, but a bit of an
advantage for a contained chemical system. From inside these
loosely-sealed chambers raw materials could be accessed and kept
away from poachers temporarily, and chemical systems could evolve
with some room to move without everything floating away from each
other. At this point Life, in its so-called protocells, would start to have a noticeable
resemblance to todays simplest organisms: a
membrane-enclosed soup of active proteins, made using some form of
nucleic acids, interacting with the environment, pinching off
offspring, and struggling to compete, winners continuing a family
line and losers dying out.

LIFE.

Except...

WHAT TO DO WHEN THE
FOOD RUNS OUT?

This worldwide ocean of competing heterotrophs had
two sources of fuel to run on; the original primordial soup,
quickly being consumed with no way to replenish it, and other
early organisms. This is not a formula for long-term
success, and unless a system for making new fuel emerged, the
limited resources would eventually run out. One wonders on
how many planets across the universe this course into a dead end
may have happened. Perhaps life on Mars was doomed even before the
ancient oceans dried up...

But that didnt happen on Earth, because autotrophs,
they who were eventually to become the whole basis of the food
chain, evolved. Most likely, the first autotrophs were able to
assemble the hot chemicals from hydrothermal vents in especially efficient
ways, a process called chemosynthesis. Simple
organisms like this can be found today in hydrothermal vents and
some hot springs.

But how to get to photosynthesis,
a system that also uses energy from the environment to construct
fuel molecules, but one using light, a very different form of
energy than heat? One possibility is that, in order to stay
near their vent homes (which are like oases in a desert, you dont
want to wander too far), some organisms developed ways to detect
the faint glow the vents produce. Bacteria still living near vents
have been found to do this. Once you have a
chemosynthesis system and a light-reactive system in the same
organism, its easier to imagine a system that could use light as
an energy source for synthesis, and in fact, a tiny fraction of bacteria
in todays vent communities
do photosynthesis using this weak light. This would be a minor
advantage there in the mostly-dark, but a huge
advantage to those bacteria swept upward: chemosynthesis-based ecosystems, even in the
early earth, would have been few and far between, but the entire
surface of the oceans would have access to light.

PHOTOSYNTHESIS
CHANGES THE WORLD

The basic process of photosynthesis combines water
molecules and carbon dioxide molecules to make simple sugar
molecules, usable as fuel and as a structural building-block in
plants. The process needs light to work and releases oxygen
as a bi-product. The early earth would have had little
oxygen, or at least free oxygen, because the oxygen present
would have been "tied up" in molecules such as water,
carbon dioxide, and a long list of mineral compounds in
rock. Oxygen is good at combining with other materials,
which would have made it a potential poison to complex
living systems. As photosynthetic organisms flourished,
systems to resist the damaging effects of free oxygen had to
evolve as well (our own bodies have multiple systems in place to
resist oxygen damage). The environment itself changed:
a layer of sedimentary rock from this period shows that the oxygen
combined with an ocean full of dissolved iron, settling a layer of
iron oxide (rust) into the sediments, an indicator that the
chemistry of the oceans themselves were changing. Oxygen left the
oceans (it doesnt dissolve particularly well in water) and built
up in the atmosphere, eventually rising to much higher levels than
can be found in the waters.

Respiration is a system by which fuel
molecules from food are broken down and the energy used to build the fuel molecules that power
cells, mostly in the form of a molecule called ATP (Adenosine
triphosphate). Various approaches would have
existed long before the rise of photosynthesis, but the buildup of
oxygen and wide availability of the simple sugar (glucose)
produced by photosynthesis favored the rise of a particularly
efficient respiration system that was almost a mirror image of
photosynthesis: aerobic
respiration.

So at this stage of Earths history, the oceans
would have been full of photosynthesizing prokaryotes, aerobic
prokaryotes, and predatory prokaryotes feeding off the
others. And a truly diversified world was about to get even more so...

COMPARTMENTALIZATION
AND ENDOSYMBIOSIS

As some cells got more complicated, subdividing the
cell into smaller specialized chambers with their own particular
chemistry could increase the efficiency of certain processes,
especially the process by which DNA code was used to make proteins. At some point, this DNA
processing was put into a separate specialized room, the nucleus, and eukaryotes
evolved - larger, efficient, able to gobble up smaller prokaryotes
but probably not able to generate the same huge numbers.

An ingested prokaryote can be digested and
absorbed, but sometimes they might be more useful confined and
alive. A photosynthetic prokaryote could make food in the
sun for its captor, and an aerobic prokaryote could help the larger cell better utilize
the food it took in. The prokaryotes benefited, too:
beyond not being digested, becoming part of the larger predatory
cells certainly reduced their potential as prey. This type
of mutual-benefit relationship is called a symbiosis,and that
the absorbed prey might be used rather than digested is
not only reasonable, it can be
found in a few of todays Protistans.
The vast majority of present-day eukaryote cells contain aerobic
respiration chambers called mitochondria that
structurally
resemble aerobic bacteria, even down to having the
remnants of bacterial chromosomes in them, and eukaryote plant
cells contain photosynthesizing chloroplasts with
similar resemblances to a type of photosynthetic prokaryote.
That these structures
began in the way described here is known as
the endosymbiont theory,proposed in the late
1960s by Lynn Margulis, then at Boston University, and eventually
widely accepted.
Other eukaryote structures might have endosymbiont origins, but the evidence for those is more
controversial.

So the worlds oceans became a mixture of prokaryotes and simple
eukaryotes for a very long time. Eventually, something that
eukaryotes can sometimes do but prokaryotes almost never do led to
the next major step of Life on Earth...

BECOMING
MULTICELLULAR

Most of the noticeable life in our world is big,
multicellular. The trip to multicellularity undoubtedly went
through the colonialism stage: colonial
organisms are made up of individuals that are capable of living
independently, but join together and then specialize in different
jobs. When unicellular organisms do this, you have a
multicellular organism that can be split up and still survive. An evolutionary progression to cells that can live
together, specialize, and become so dependent on each other that
they can no longer live apart is not much of a leap.

Prokaryotes can and often do live in groupings -
a type of mineralized bacterial surf structure, stromatolites,
shows up in very ancient fossils and can be found living today - but the
individuals rarely specialize. Its more like they "hang
out" together. Eukaryotes, perhaps because with nuclei
they are better able to control what genes get expressed in a
particular cell, seem to have a talent for specialization within a group.

For a very long time, the only multicellular
forms in the fossil record were algae, barely above the colonial
stage. Multicellularity was an advantage for plants, but the
potentials seemed limited to floating mats or attached strands in
the shallows. But when animals evolved
multicellularity, it
was like the evolutionary floodgates opened up. Over an
incredibly brief time, the world became full of swimming and
crawling eating machines. In a world where life had existed
for 3 billion years, many types of complex multicelled animals
"appear" in the fossil record over a period that might
be as short as 40 million years, a period called the Cambrian
Explosion. With maybe one exception, every
known phylum of animals evolved during this period (and several that
no longer exist), including the first
known fossil examples of our lineage.

The war was on. Plants seemed to be able
to deal with larger plant-eaters with few obvious
adaptations: algae remained relatively simple. But
animals adapted in a range of ways. They got bigger and nastier, or smaller and quicker,
or more
protected; think of any adaptation that provides an advantage
in a world of animals, and it appeared during the Cambrian
Explosion. Except one...

"SO WHERE ELSE
CAN WE GO?"
THE MOVEMENT ONTO LAND

The water is a great place for a living thing,
since they depend on water to float their molecules and support
their chemistry. Life evolved in the oceans and filled them
wherever there was enough light to photosynthesize or food to
eat. But there were niches going unused, up out of the
water, on the bare land of the continents. How could the potential
niches there be reached and filled?

The land environment would have had several
significant differences from the water environment that would need
to be adapted to:

- NOTHING WAS ALIVE UP THERE. For
the first, pioneer organisms (this term is applied
to the newcomers in any "new" environment), they needed
to deal with an environment devoid of life and
nutrient-poor. Animals might use it as a place to avoid
predators, but would need to return to the water to feed.
Plants had a trickier obstacle: the light, water, and carbon
dioxide they needed for photosynthesis might be available, but
other nutrients for making molecules such as proteins would not
be. It is quite likely that plants would not have been able
to move onto the land without symbioses established with fungi and
bacteria to help them get the materials they needed.
- WATER EVAPORATES IN THE AIR. The
water content of cells is critical to the function of cells - if
too much is lost or gained, the cells cease to function. A
land organism cannot lose too much water to the air or it wont
survive. But there are transitional ecosystems
that might have required adaptations usable against
evaporation: tidal zones, where organisms are
sometimes left "high and dry," as well as in pools that
might fill with rain runoff or evaporate, where resisting a similar
dilution change in the cells would be necessary; fresh
water systems, where a resistance to the inflow from very
dilute surroundings would be necessary. Our distant
ancestors, the bony fish, apparently evolved in fresh water and
developed an efficient waterproofing system to keep water from
rushing into their cells; that barrier could also be used to prevent water loss in
the air. It is quite likely that, for this and other
reasons, all life on land evolved from tidal and/or fresh water
ancestors. Of the three multicelled Kingdoms, the fungi seem
to have had the hardest time with drying, perhaps because of the
way nutrients get absorbed - its almost impossible to move
materials across a waterproofed surface - but theyve gotten by in
moister environments, in soils and in the wetness of other living
things.

- YOU CANT FLOAT IN THE AIR. The
buoyancy of water reduces the need for strong support
structures. This was especially a problem for plants, which
didnt undergo much dramatic evolution until they moved on to
land, where complex support structures and then structures to move
materials around against the force of gravity led to an explosion
of different forms. Animals had some adaptations ready to
go: muscle systems for moving quickly through the water or
across the bottom needed modification to work on land (fins and fin
supports needed
to be more leg-like in our ancestors; insect and spider
ancestors had to lighten their outer covering just to hold
themselves up), but structures used for moving across tidal flats
or in very shallow water became usable away from the water as
well.

- TEMPERATURE FLUCTUATIONS. A body
of water gains and loses heat more slowly than the air does, so
temperature changes are slower there. Temperature has a huge
effect on cellular chemistry, and only chemistry that can somehow
deal with rapid changes can be used in a land organism.
Again, tidal areas and shallow fresh water ecosystems would have
been good staging areas for developing some flexibility.
Plants, not being able to move from place to place to adjust their
temperature, had a more critical problem, and may have taken some
time to adapt to non-tropical areas.

- DIRECT SUNLIGHT. The frequencies
of energy in sunlight can cause molecules in living systems to
become unstable, as happens in the mutations that lead to human skin
cancer. Water reflects several frequencies and quickly
absorbs many more, making the problem much reduced for organisms
that live below the surface. Most land organisms have
protective pigments to keep the sunlight from penetrating and
harming them. The adaptations would also have been required
for life in tidal areas and shallow fresh water.

- MUCH MORE OXYGEN. As mentioned
earlier, the air can hold much more oxygen than water can, and
oxygen is a very reactive material (even you can be poisoned with too
much of it!). An organism cant live in the air if it cant
handle the increase in oxygen. Long-term, the higher oxygen
levels allow for much more energetic metabolisms in aerobic
animals. Even an animal like a crocodile gets such an energy
advantage from breathing air that it would never evolve a
water-breathing system again, and its difficult to understand how
anyone could ever develop a system by which a human could breathe
underwater - there just isnt enough oxygen available there.

- SPERM NEED WATER. Sexually-
reproducing animals and plants had for the most part evolved
systems where the sperm were released and had to get
themselves to the waiting egg cell by swimming. This doesnt seem like
much of a problem, but for a couple of the major land groups it was the most
difficult one to solve - long after the difficulties of water
loss, and support, and other land challenges were met, amphibians
and ferns still require open water for reproduction.

Virtually every phylum of organisms was able to
get a least a few species up onto land, although they all still
have some water-living species as well. Some researchers
hypothesize that the rise of land plants, with hard-to-break-down
carbohydrate support structures, pulled more and more carbon from the
environment. Less carbon available for aerobic respiration might
have let more oxygen accumulate, setting up an environment for
higher-metabolism, larger animals.

PROGRESSION TO
TODAYS WORLD.

Through the course of Lifes History, some
interesting things have happened:

Continental
drift. The rocks
that make up the continents mostly float like corks on heavier
molten rocks beneath. These huge corks, called continental
plates, move slowly but with huge momentum, pulling away from
each other to make things like the Atlantic Ocean (visible here,
by dragging the cursor right-and-left across the image) or colliding in
huge "fender benders" that ripple up things like the
Himalayan Mountains or the Panama bridge between the
Americas. These movement have huge effects on ocean currents, which
affect climate around
the world, driving evolutionary change as areas get wetter or
warmer or whatever.

Catastrophes. Sometimes the
world can change in an instant. Huge chunks of rock fly in
from space and smack into us, changing the weather for months or
years and flash-frying whole continents. A low-lying basin
"suddenly" connects to the ocean, and the Atlantic pours in and
forms the Mediterranean
Sea seemingly overnight. A huge volcanic eruption covers almost
a third of India with lava and spews huge amounts of
climate-changing gas and dust into the atmosphere.
Continental drift allows an invasion of new competitors across Panama into a
stable South American ecosystem over a matter of decades. Humans adjust
the environment on a huge scale to fit their preferences.
These can cause the major transitions found in the fossil record,
including a few so large that they are known as mass
extinction events; the asteroid impact that wiped out the
dinosaurs and left our tiny scavenging ancestors to take over is a
well-known one, but there have been several, and the causes are not
always known. According to recent
research, there may be a regular rise-and-fall of diversity (lots of
different species, then a drop to very few, then a rise again) on a
62-million-year cycle; this may renew interest in an older theory
about Nemesis,
a
proposed "dark star" companion of the Sun on a very
long elliptical orbit - in the original hypothesis, it was supposed to
visit every 26 million years, do bad things about the solar system with
its gravity effects, and move away, but the theory could be amended to a
62-million-year cycle.

One interesting theory
about how the worlds ecosystems are stabilized is the Gaia
Hypothesis, which says that the presence of Life itself acts
as a kind of thermostat on the planet. As an example, it is thought
that during an Ice Age, there is less run-off from the continents into the
oceans (less liquid precipitation and melt); less run-off means
fewer nutrients for the oceans algae, which means less photosynthetic
processing of carbon dioxide, a heat-trapping greenhouse gas. More
carbon dioxide traps more heat, raising the temperature and ending the Ice
Age - then run-off increases, plants rebound, absorb more carbon dioxide, and keep the greenhouse
warming from getting too extreme. This may be only a piece of the
story, but it may explain why the Earth has stayed within a limited range
of surface temperatures for several billion years, while the sun has
brightened.

One cause sometimes proposed is disease,
but this is very very unlikely as a major player for a couple of
reasons. For one, diseases tend to adapt to particular hosts
- a disease that can affect a wide variety of organisms is
extremely rare, and even then the effects vary because each type
of host is a unique ecosystem. More importantly, however, is
that diseases are caused by evolving organisms, and the more
successful individuals are not the deadly ones, but
the ones that keep the host semi-well and moving around to spread offspring. Except in tiny systems or small populations,
diseases get less damaging as they spread and so are unlikely
candidates for causing widespread carnage.

Speaking of disease, there is a
possibility that viruses,which are tiny complexes, often
non-cellular, which invade cells and convert at least part of their
DNA-to-protein equipment to virus manufacturing, are leftovers of that
precellular life that existed in the primordial soup.

KEY
CONCEPTS -Click on term to go to it in the
text.
Terms are in the order they appear.